1. Field of the Invention
This invention relates to a wire shield structure and a wire shielding method suitably used to shield a wire harness or the like installed in an automobile.
2. Background
With the development of automatic operations, audio and visual equipments and a navigation device are mounted on a current automobile, and naturally many wire harnesses, interconnecting the electronic equipment, and many power cords are used. Travel control signals and so on are in the digital form, and therefore these signals can be easily affected by electrical noises, and when such electrical noises are superimposed on the control signal, an accident is liable to occur.
Therefore, electromagnetic shielding, described below, has been effected in order to prevent the influence of electrical noises and also to prevent radiation of electrical noises.
A zipper tube, having a shielding effect, is disclosed in Unexamined Japanese Utility Model Publication No. Sho. 56-158627. This will briefly be described with reference to FIGS. 10 and 11. A metal foil 32 for shielding purposes is secured to a flexible, sheet-like insulating member 31, and fastening portions 31a and 31b are formed respectively at opposite ends of the insulating member 31, and a drain wire 33 is sewn to the metal foil 32 by a metal strand wire 34.
In use, the shielding metal foil 32 and the sheet-like insulating member 31 are wound on a wire harness (a bundle of wires), not shown, and extension portions 31c and 31d of the insulating member are caused to overlap each other as shown in FIG. 11, and the fastening portions 31a and 31b are fastened together outside of these extension portions, thereby attaching the zipper tube to the wire harness. Then, the drain wire 33 and the metal strand wire 34 are grounded, thereby shielding the wire harness.
Unexamined Japanese Utility Model Publication No. Hei. 1-121998 discloses a cable protector having a shielding effect. This will briefly be described with reference to FIG. 12. A resin band 41 is spirally wound on a bundle of wires 42, and a shielding aluminum foil is affixed to an inner surface of the resin band 41 (which is to be contacted with the bundle of wires 42) over an entire length thereof. A plurality of mounting holes 43 are formed through the resin band 41, and are spaced a predetermined distance from one another along the length of the resin band 41.
In use, the resin band 41 is wound on the bundle of wires 42, with the aluminum foil contacted with this wire bundle, and the resin band 41 is cut into a required length, and the resin band 41 is fastened to a chassis 45 by screws 44 passing respectively through the mounting holes 43 provided respectively at opposite ends of the resin band 41. As a result, the aluminum foil is connected to the chassis 45, and therefore the wires 42 are shielded by the aluminum foil, and are fixed to the chassis 45.
Unexamined Japanese Utility Model Publication No. Sho. 62-152629 discloses a wire covering material having a shielding effect as described above. This wire covering material includes a metal foil contained in a long, spiral insulating member, and in use, the insulating member is wound on wires. The metal foil is grounded using a grommet driven into the insulating member to contact the metal foil, an earth wire connected to this grommet, and an earth terminal connected to this earth wire.
Unexamined Japanese Utility Model Publication No. Hei. 3-128998 discloses an electromagnetic shield member for wiring which has a shielding effect. This electromagnetic shield member for wiring includes an electromagnetic shield element (which is in the form of a copper wire net) provided in a flat, spiral tube made of vinyl chloride or the like, and one end of this electromagnetic shield element is extended from the spiral tube, and is connected to an earth connection terminal.
In use, the spiral tube is wound on a bundle of wires, and the earth connection terminal is fixedly connected to a chassis or the like by a screw, thereby effecting the electromagnetic shielding.
In addition to the above shield structures, a shield structure of a construction as shown in FIGS. 13 and 14 is known. In this shield structure 51, an insulating layer 52 and a metal foil layer 53 are connected together in layers, and an earth wire 54 is connected to the metal foil layer 53, and a band-like adhesive layer 55 is formed on the surface of the metal foil layer 53. In use, this shield structure is wound on a bundle of wires 56 as shown in FIG. 14, and holds this wire bundle by the adhesive layer 55. Then, an earth terminal 57, connected to the earth wire 54, is fixedly secured to a chassis or the like by a screw, thereby grounding the metal foil layer 53 so as to shield the wires 56 from electromagnetic waves.
However, the above-mentioned wire shield structures have the following problems.
Each of the zipper tube shown in FIGS. 10 and 11, the wire shield structure 51 shown in FIGS. 13 and 14 and the wire covering material disclosed in Unexamined Japanese Utility Model Publication No. Sho. 62-152629 has the earth wire, and the joint between the metal foil and the earth wire is liable to be destroyed by an external force. And besides, the number of the component parts is large, and the time and labor required for the assembling operation are increased, and this increases the manufacturing cost.
Moreover, there is encountered a serious problem that the use of the earth wire adversely affects the shielding performance. Incidentally, in a test in which there was used an earth wire having a diameter of 0.5 mm.sup.2 and a length of 200 mm, and the degree of attenuation was measured varying the frequency (500 KHz, 1 MHz, 2 MHz), the attenuation of only about 16 dB was achieved.
When the length of the earth wire was reduced into 50 mm, the attenuation was improved to 23 to 24 dB, and therefore it has become clear that the length of the earth wire is an important factor in the improvement of the shield effect.
In the cable protector shown in FIG. 12 and the electromagnetic shield member for wiring disclosed in Unexamined Japanese Utility Model Publication No. Hei. 3-128998, part of the metal foil serves as an earth wire, and the metal foil, when exposed, is liable to be damaged, and when such damage occurs, the shielding performance is degraded.
And besides, if the reverse surface facing away from the grounding surface is connected when grounding the metal foil, the metal foil fails to be grounded, and in this case the shielding effect is not obtained.
In the case of the cable protector shown in FIG. 12, the shielded wire bundle has those portion which are not actually shielded, and the wires are liable to be affected by noises due to electromagnetic waves, and also there is a risk that the noises are radiated, and therefore the satisfactory electromagnetic shielding is not obtained.